Pure-H2O-fed Electrocatalytic CO2 Reduction to C2H4 Beyond 1000-hour Stability

ABSTRACT

The present disclosure provides a pure-H 2 O-fed membrane-electrode assembly (MEA) electrolysis system for electrocatalytic CO 2  reduction (ECO 2 R) to ethylene (C 2 H 4 ) and C 2+  compounds under an industrial applicable continuous flow condition with at least 1000-hour lifetime, and fabrication method thereof.

TECHNICAL FIELD

The present invention relates to a pure-H₂O-fed electrolysis system forelectrocatalytic CO₂ reduction (ECO₂R). In particular, the presentinvention provides a pure-H₂O-fed membrane-electrode assembly (MEA)electrolysis system under an industrial applicable continuous flowcondition for ECO₂R-to-C₂H₄/C₂₊ compounds using a high-performancestep-facet-rich Cu (SF-Cu) catalyst to result in a lifetime of over 1000hours.

BACKGROUND

ECO₂R has wide variety of applications, for example, formation ofhigh-value chemicals and feedstocks using renewable electricity, whichcould decouple the chemical and fuel productions from fossil fuels andthus close the carbon loop, offering possibilities to mitigategreenhouse gas emissions. Optimizing selectivity, i.e., Faradaicefficiency (FE), of catalysts for high-value products such as CO, HCOOH,and C₂H₄, increasing their productivity (current density), and loweringoverpotentials of the reduction reactions have become priorities andbeen with some significant advances. However, one of the problems is thesystem stability. Formation and crossover of carbonate in both alkalineand neutral electrolytes during electrolysis result in additional energyconsumption and CO₂ losses, lowering the durability of ECO₂R.

Another problem is the strong local alkaline conditions present in ECO₂Rcauses a major fraction of the input CO₂ to react with the OH⁻ toproduce CO₃ ²⁻ rather than being reduced into carbon-based products,lowering the reduction efficiency. Some recent studies showed thatregenerating CO₂ from CO₃ ²⁻ requires more than 230 kJ/mol in acalcination system, but the energy stored by that ECO₂R was just 100-130kJ/mol of electrons, depending on different products, which indicatedthat the net energy balance in the alkaline/neutral electrolyte wasnegative.

In principle, since each electron from ECO₂R can consume 1 OH⁻equivalent, taking ECO₂R to C₂H₄ in the alkaline/neutral electrolyte asan example, forming 1 C₂H₄ molecule will produce 12 OH⁻ that can reactwith 6 CO₂ into 6 CO₃ ²⁻ (Eq. 1 and 2):

Cathode: 2CO₂+8H₂O+12e ⁻→C₂H₄+12OH⁻  (1)

12OH⁻+6CO₂→6CO₃ ²⁻+6H₂O  (2).

In theory, a large amount of carbonate would precipitate in the gasdiffusion electrode (GDE) and CO₂ flow channel of the cell, blocking CO₂transport, accelerating electrolyte flooding and eventually shuttingdown the ECO₂R reaction, which leads to poor ECO₂R stability. As aresult, the theoretically maximum carbon efficiency of ECO₂R-to-C₂H₄ is25% and it is even far lower than this theoretical limit in the actualelectrolysis process where the cathodic catalyst is less efficient orthe strong alkaline electrolyte is involved. So far, the stability ofECO₂R-to-C₂H₄ in the conventional flow cell ormembrane-electrode-assembly (MEA) cell with the alkaline/neutralelectrolyte is generally less than 200 hours.

In an anion-transporting cell assembled with the anion exchange membrane(AEM), CO₃ ²⁻ formed at the cathode will be transported to the anode tobe protonated and release CO₂ and OH⁻. This process can consume up to˜70% of the energy input for the ECO₂R reaction. Therefore, theconventional electrolysis system of ECO₂R needs to be operated in strongacid (pH<1) in a flow cell to eliminate the carbonate formation andcrossover at the expense of a portion of ECO₂R products. However, thisacidic-electrolysis system cannot satisfy the MEA configuration, forexample, as shown in FIGS. 1A and 1B.

A need therefore exists for an improve MEA cell system that eliminatesor at least diminishes the disadvantages and problems described above.

SUMMARY OF INVENTION

Accordingly, the present disclosure provides a pure-H₂O-fed MEAelectrolysis system on a high-performance step-facet-rich Cu (SF-Cu)catalyst with fast kinetics for ECO₂R-to-C₂H₄. The system integrates theAEM and proton exchange membrane (PEM) to selectively transport theelectrogenerated OH⁻ and H⁺, respectively. The system does not onlyboost the pure-H₂O-fed ECO₂R reaction activity by increasing the localpH on the cathode catalyst surface but also eliminates carbonateformation and crossover, leading to prolonged stability.

An aspect of the present invention provides a pure-H₂O-fedmembrane-electrode assembly electrolysis system for electrocatalytic CO₂reduction to ethylene and C₂₊ compounds including ethanol, propanol, andacetic acid under an industrial applicable continuous flow conditionwith at least 1000-hour lifetime, where the system includes one or moremembrane-electrode assemblies, and each of the membrane-electrodeassemblies include:

-   -   an anode;    -   a cathode;    -   an anion exchange membrane;    -   a proton exchange membrane;    -   a step-facet-rich copper catalyst disposed at the cathode; and    -   an electrolyte,    -   where:    -   the cathode is arranged in contact with the anion exchange        membrane;    -   the anode is arranged in contact with the proton exchange        membrane;    -   the anion exchange membrane and proton exchange membrane are        arranged in contact with each other;    -   the electrolyte is selected from pure H₂O as proton source for        the electrocatalytic CO₂ reduction at the cathode under a        forward bias mode of the system;    -   the anion exchange membrane is selected from alkaline anion        exchange membrane or bipolar membrane; and    -   the proton exchange membrane is selected from acidic proton        exchange membrane or bipolar membrane.

In certain embodiments, the cathode is selected from a gas diffusionelectrode deposited with at least a layer of the step-facet-rich coppercatalyst.

Preferably, the cathode is a carbon paper with a microporous carbon gasdiffusion layer coated with the step-facet-rich copper catalyst.

In certain embodiments, the anode is selected from titanium fiber feltsupported by one or more of platinum, iridium, ruthenium, and palladium,and any oxide or alloy thereof.

Preferably, the anode is a titanium fiber felt sputtered by platinumthereon.

In other embodiments, the anode can be a titanium fiber felt sputteredby iridium, ruthenium, and palladium, and any oxide or alloy thereof.

In some other embodiments, the anode can be a carbon paper supported bythe one or more of platinum, iridium, ruthenium, and palladium, and anyoxide or alloy thereof.

In certain embodiments, the electrocatalytic CO₂ reduction is conductedat a temperature of about 60° C. or lower but above room temperature.

Preferably, the electrocatalytic CO₂ reduction is conducted at about 60°C.

In certain embodiments, the alkaline anion exchange membrane is an anionexchange membrane made of N-methylimidazolium-functionalized styrenepolymer.

Preferably, the alkaline anion exchange membrane is an anion exchangemembrane made of N-methylimidazolium-functionalized styrene polymer witha thickness of about 0.002 inches.

In certain embodiments, the acidic proton exchange membrane is a protonexchange membrane made oftetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer.

Preferably, the acidic proton exchange membrane is a proton exchangemembrane made oftetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer with a thickness of about 0.007 inches and an equivalentweight of about 1100 g/mol.

In certain embodiments, the step-facet-rich copper catalyst has avariable surface atom coordination number from 4 to 9 at either one orboth of Cu (111) and Cu (100) exposed facets.

In certain embodiments, the step-facet-rich copper catalyst has avariable surface tensile strain within 10% of an initial tensile strainthereof measured at room temperature.

In certain embodiments, at least six of the membrane-electrodeassemblies are stacked together.

In certain embodiments, up to about 50% of Faradaic efficiency towardsethylene with a carbon dioxide-to-ethylene conversion efficiency ofabout 39% is achieved when a total current of 10 A is supplied acrossthe at least six membrane-electrode assemblies through two conductivesubstrates sandwiching the stack of the at least six membrane-electrodeassemblies with a total geometrical area of 30 cm².

In other embodiments, the total geometrical area of the one or more ofthe membrane-electrode assemblies is variable subject to the demand forCO₂ reduction, current density, size of the electrolysis cell,conductivity of the electrodes, membranes and substrates thereof, etc.

In some other embodiments, the electrolysis cell includes a stack ofmultiple membrane-electrode assemblies or a single membrane-electrodeassembly with a relatively larger geometrical area, or both.

Preferably, the stack of multiple membrane-electrode assemblies isselected over the single membrane-electrode assembly in an industrialapplicable continuous flow condition since the stack configuration isrelatively more flexible and easier to be scaled up or down according tothe demand for CO₂ reduction and compatibility to other equipment in anindustrial plant or setting.

Another aspect of the present invention provides a method forfabricating a pure-H₂O-fed membrane-electrode assembly electrolysissystem for electrocatalytic CO₂ reduction to ethylene and C₂₊ compoundsincluding ethanol, propanol, and acetic acid with at least 1000-hourlifetime, where the method includes:

-   -   providing a step-facet-rich copper catalyst;    -   preparing a step-facet-rich copper catalyst-containing ink        composition for forming a cathode with the step-facet-rich        copper catalyst thereon;    -   forming the cathode with the step-facet-rich copper catalyst        thereon;    -   preparing an anode-forming mixture for forming an anode;    -   forming the anode from the anode-forming mixture supporting an        anode material;    -   providing an alkaline anion exchange membrane and an acidic        proton exchange membrane between said cathode and anode, where        the alkaline anion exchange membrane is arranged in contact with        the cathode; the acidic proton exchange membrane is arranged in        contact with the anode; and the alkaline exchange membrane and        acidic proton exchange membrane are in contact with each other,        thereby forming a multi-layered structure of the        membrane-electrode assembly;    -   sandwiching one or more of the membrane-electrode assemblies        with two conductive substrates;    -   feeding pure H₂O as an electrolyte into a container containing        the one or more of the membrane-electrode assemblies being        sandwiched between the conductive substrate;    -   providing a power supply to the one or more of the        membrane-electrode assemblies through the two conductive        substrates;    -   maintaining the electrolyte at a temperature sufficient for the        electrocatalytic CO₂ reduction to ethylene to last for at least        1000 hours with no dominant hydrogen evolution reaction.

In certain embodiments, the step-facet-rich copper catalyst is providedby:

-   -   dissolving copper chloride and octadecylamine into squalene at        about 80° C. under an argon atmosphere for about 0.5 hours until        a copper-based stock solution is formed;    -   mixing oleylamine and trioctylphosphine under heating the        mixture to about 200° C. at the argon atmosphere with vigorous        agitation to form a mixture;    -   injecting the copper-based stock solution into the mixture at        about 200° C. and maintained for about 5 hours to form a        reaction mixture;    -   cooling the reaction mixture naturally, centrifuging the cooled        reaction mixture, followed by washing with an organic solution        for a few times; and    -   removing supernatant after said washing and blow drying pellet        with argon gas under room temperature to obtain the        step-facet-rich copper catalyst in solid form.

In certain embodiments, at about 1:2 weight ratio of copper chloride tooctadecylamine are dissolved in squalene.

In certain embodiments, about 20:1 volume ratio of oleylamine totrioctylphosphine are mixed under heating at 200° C. under argon gas.

In certain embodiments, the organic solution for washing thecentrifuged, cooled reaction mixture is n-hexane.

In certain embodiments, the cathode is formed with the step-facet-richcopper catalyst coated thereon by:

-   -   dispersing the solid step-facet-rich copper catalyst into a        mixed solution containing water, isopropyl alcohol and an        alkaline ionomer solution;    -   mixing the solid step-facet-rich copper catalyst with the mixed        solution by sonication for about an hour until the        step-facet-rich copper catalyst-containing ink composition is        formed;    -   coating the step-facet-rich copper catalyst-containing ink        composition onto a carbon paper with a microporous carbon gas        diffusion layer;    -   drying the coated step-facet-rich copper catalyst-containing ink        composition on the carbon paper in vacuum for about an hour.

In certain embodiments, the anode is formed from a titanium fiber feltsupported by the anode forming mixture comprising one or more ofplatinum, iridium, ruthenium, and palladium, and any oxide or alloythereof.

In certain embodiments, the alkaline anion exchange membrane is selectedfrom an anion exchange membrane made ofN-methylimidazolium-functionalized styrene polymer with a thickness ofabout 0.002 inches; the acidic proton exchange membrane is selected froma proton exchange membrane made oftetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer with a thickness of about 0.007 inches and equivalent weightof 1100 g/mol.

In certain embodiments, at least six of the membrane-electrodeassemblies are stacked with each other and sandwiched between the twoconductive substrates; the electrolyte temperature is maintained atabout 60° C.

In certain embodiments, the at least six of the membrane-electrodeassemblies have a total geometrical area of about 30 cm².

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used as an aid in determining the scope of the claimed subjectmatter. Other aspects of the present invention are disclosed asillustrated by the embodiments hereinafter.

BRIEF DESCRIPTION OF DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

The appended drawings, where like reference numerals refer to identicalor functionally similar elements, contain figures of certain embodimentsto further illustrate and clarify the above and other aspects,advantages and features of the present invention. It will be appreciatedthat these drawings depict embodiments of the invention and are notintended to limit its scope. The invention will be described andexplained with additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1A shows a comparison of the stability of ECO₂R-to-C₂H₄ on Cu-basedcatalysts in the flow cell or MEA cell according to certain embodimentsof the present invention with that of conventional systems according tocertain literatures;

FIG. 1B shows a result of a long-stability performance test onECO₂R-to-C₂H₄ on SF-Cu in a pure-H₂O-fed MEA-cell stack containing 6 MEAcells at a constant current of 10 A according to certain embodiments ofthe present invention, where the total cathode electrode area is set tobe 30 cm² and the reaction temperature is set to be 60° C.;

FIG. 2A shows an SEM image of the SF-Cu catalyst according to certainembodiments of the present invention;

FIG. 2B shows a HRTEM image of the SF-Cu according to certainembodiments of the present invention, revealing abundant stacking faults(yellow rectangular box marked with D);

FIG. 2C shows a HRTEM image of the SF-Cu in certain embodiments of thepresent invention, revealing interlaced grain (twin) boundaries (yellowrectangular box marked with E);

FIG. 2D shows an atomic-resolution HAADF-STEM image of stacking faultsfrom the selected area marked with D in the rectangular box as shown inFIG. 2B; yellow lines highlight stacking faults;

FIG. 2E shows an atomic-resolution HAADF-STEM image of twin boundariesfrom the selected area marked with E in the rectangular box as shown inFIG. 2C; yellow lines highlight five-fold twin boundaries;

FIG. 2F shows an atomic-resolution HAADF-STEM image of surfacestep-facets of the SF-Cu induced by a stacking fault and a twinboundary, where both the stacking fault and twin boundary along {111}planes are indicated by white dashed lines;

FIG. 2G shows geometric-phase analysis (GPA) strain mapping of tensilestrain (C) near the surface exits of the stacking fault and the twinboundary as shown in FIG. 2F using the lattice far from defects as areference (zero strain), where the tensile strain as measured isperpendicular to the {111} plane along which the stacking fault and twinboundary align with each other;

FIG. 3 shows in-situ heating characterization on different states ofSF-Cu: (A and B) TEM images of the pristine SF-Cu (Before) and the SF-Cuheated at 650° C. for 20 min (After); (C and D) HRTEM images of thepristine SF-Cu (Before) and the SF-Cu heated at 650° C. for 20 min(After);

FIG. 4 shows SEM images and size distribution of different catalystnanoparticles: (A-C) SF-Cu; (D-F) Cu-250; (G-I) Cu-350; (J-L) Cu-450;

FIG. 5A shows X-ray absorption near edge structure (XANES) spectra ofSF-Cu, Cu-250, Cu-350, Cu-450 and the standard Cu foil referencerecorded at the Cu K-edge; values are means; error bars indicate SD (n=3replicates);

FIG. 5B shows Fourier transform of Cu K-edge EXAFS spectra of SF-Cu,Cu-250, Cu-350, Cu-450 and the standard Cu foil reference; values aremeans; error bars indicate SD (n=3 replicates);

FIG. 5C shows FEs toward ECO₂R products on SF-Cu under a range ofapplied potentials in a flow cell with 1 M KOH as the electrolyte;values are means; error bars indicate SD (n=3 replicates);

FIG. 5D shows comparisons of FEs toward C₂H₄ of SF-Cu, Cu-250, Cu-350and Cu-450; values are means; error bars indicate SD (n=3 replicates);

FIG. 5E shows comparisons of partial current densities (J) toward C₂H₄of SF-Cu, Cu-250, Cu-350 and Cu-450; values are means; error barsindicate SD (n=3 replicates);

FIG. 5F shows the relationship between the tensile strain, CN and thepeak j_(C2); values are means; error bars indicate SD (n=3 replicates);

FIG. 6A schematically depicts reaction scheme of ECO₂R in thepure-H₂O-fed MEA cell assembled with AEM and PEM according to certainembodiments of the present invention;

FIG. 6B shows FEs toward ECO₂R products under a range of applied currentdensities in the MEA cell with pure H₂O as the electrolyte, and thecorresponding cell voltages without iR compensation; Pt/Ti is used asthe anode electrode and the reaction temperature is set at 60° C.;

FIG. 6C schematically depicts the MEA-cell stack containing 6 MEA cellsfor ECO₂R reaction according to certain embodiments of the presentinvention;

FIG. 6D shows stability monitoring of the MEA-cell stack containing 6MEA cells at a constant current of 10 A according to certain embodimentsof the present invention, where an inset shows a digital photograph ofthe monitoring system;

FIG. 7 shows an X-ray diffraction (XRD) patterns of SF-Cu, Cu-250,Cu-350 and Cu-450 on the carbon paper, and bare carbon paper;

FIG. 8 shows an X-ray photoelectron spectroscopy (XPS) spectra of SF-Cu,Cu-250, Cu-350 and Cu-450: (A) Cu 2p XPS spectra. (B) Cu LMM Augerspectra. (C) O is XPS spectra;

FIG. 9 shows an X-ray absorption spectroscopy (XAS) spectra of SF-Cu,Cu-250, Cu-350 and Cu-450, and the standard Cu foil, CuO and Cu₂Oreferences: (A) Cu K-edge XANES spectra; (B) Fourier transform of CuK-edge extended X-ray absorption fine structure (EXAFS) spectra;

FIG. 10 shows Cu K-edge EXAFS fitting curves at R and q space,respectively: (A1-A2) Cu foil reference; (B1-B2) SF-Cu; (C1-C2) Cu-250;(D1-D2) Cu-350; (E1-E2) Cu-450;

FIG. 11 shows two-dimensional plots of wavelet transform EXAFS (2D WTEXAFS): (A) Standard Cu foil reference; (B) SF-Cu; (C) Cu-250; (D)Cu-350; (E) Cu-450; (F) Standard Cu₂O reference; (G) Standard CuOreference;

FIG. 12 shows exposed facets of SF-Cu determined by lead underpotentialdeposition (Pd-UPD);

FIG. 13 shows atomic models with different CNs on Cu (111) (side view,top view and Cu site with the different CN): (A) The perfect Cu (111),CN: 9; (B-D) CN: 8, 7 and 6, respectively;

FIG. 14 shows atomic models with different CNs on Cu (111) (side view,top view and Cu site with the different CN): (A-F) CN: 7, 7, 6, 5, 5 and5, respectively;

FIG. 15 shows Atomic models with different CNs on Cu (100) (side view,top view and Cu site with the different CN): (A) The perfect Cu(100),CN: 8. (B and C) CN: 7 and 6, respectively;

FIG. 16 shows atomic models with different CNs on Cu(100) (side view,top view and Cu site with the different CN): (A-D) CN: 6, 6, 5 and 4,respectively;

FIG. 17 shows the total current density of ECO₂R under different appliedpotentials over SF-Cu in a flow cell with 1 M KOH as the electrolyte;values are means; error bars indicate SD (n=3 replicates);

FIG. 18 shows ECO₂R performance on Cu-250 under different appliedpotentials in a flow cell with 1 M KOH as the electrolyte: (A) FEstowards ECO₂R products; (B) total current density; values are means;error bars indicate SD (n=3 replicates);

FIG. 19 shows ECO₂R performance on Cu-350 under different appliedpotentials in a flow cell with 1 M KOH as the electrolyte: (A) FEstowards ECO₂R products; (B) total current density; values are means;error bars indicate SD (n=3 replicates);

FIG. 20 shows ECO₂R performance on Cu-450 under different appliedpotentials in a flow cell with 1 M KOH as the electrolyte: (A) FEstowards ECO₂R products; (B) total current density; values are means;error bars indicate SD (n=3 replicates);

FIG. 21 shows a comparison of the total current densities on SF-Cu,Cu-250, Cu-350 and Cu-450 for the ECO₂R reaction in a flow cell with 1 MKOH as the electrolyte under a range of applied potentials;

FIG. 22 shows a comparison of FEs and partial current densities towardC₂₊ products on SF-Cu, Cu-250, Cu-350 and Cu-450 for the ECO₂R reactionin a flow cell with 1 M KOH as the electrolyte under a range of appliedpotentials: (A) FEs toward C₂₊ products; (B) Partial current densitiesof C₂₊;

FIG. 23 shows relationships between (A) strain, CN and the peakj_(C2H4); and (B) strain, CN and the j_(without H2) for the ECO₂Rreaction in a flow cell with 1 M KOH as the electrolyte;

FIG. 24 shows a relationship between strain, CN and the j_(H2) under thepeak ECO₂R performance in a flow cell with 1 M KOH as the electrolyte;

FIG. 25 shows SEM images (A-C) and size distribution (D) of theoxide-derived Cu nanoparticles;

FIG. 26 shows XRD patterns of SF-Cu, oxide-derived Cu on the carbonpaper, and bare carbon paper;

FIG. 27 shows XPS spectra of the oxide-derived Cu: (A) Cu 2p XPSspectrum; (B) Cu LMM Auger spectrum; (C) O 1s XPS spectrum;

FIG. 28 shows ECO₂R performance on the oxide-derived Cu under differentapplied potentials in 1 M KOH: (A) FEs toward ECO₂R products; (B) Totalcurrent density; values are means; error bars indicate SD (n=3replicates);

FIG. 29 shows a comparison of total current densities on SF-Cu andoxide-derived Cu for the ECO₂R reaction in a flow cell with 1 M KOH asthe electrolyte under a range of applied potentials;

FIG. 30 shows comparisons of the ECO₂R performance on SF-Cu andoxide-derived Cu in a flow cell with 1 M KOH as the electrolyte under arange of applied potentials: (A and C) comparisons of FEs toward C₂₊ andC₂H₄, respectively; (B and D) comparisons of partial current densitiesof C₂₊ and C₂H₄, respectively;

FIG. 31 shows ECO₂R performance on SF-Cu and SF-Cu/PMMA in a flow cellwith 1 M H₃PO₄: (A) The FE and total current density on SF-Cu under arange of applied potentials, no ECO₂R product, just H₂; (B) The FE andtotal current density on SF-Cu/PMMA under a range of applied potentials,no ECO₂R products, just H₂, where 1 M H₃PO₄ is used as the electrolyte;values are means, and error bars indicate SD (n=3 replicates); (C) SEMimage of the surface of SF-Cu/PMMA; (D) Cross-sectional SEM image ofSF-Cu/PMMA;

FIG. 32 shows ECO₂R performance on SF-Cu/PMMA in a flow cell with 1 MH₃PO₄ containing 3 M KCl as the catholyte and 1 M H₃PO₄ as the anolyte:(A) FEs towards ECO₂R products under a range of applied potentials; (B)corresponding total current density under a range of applied potentials;values are means; error bars indicate SD (n=3 replicates);

FIG. 33 shows ECO₂R performance on SF-Cu/PMMA in a flow cell with 1 MH₃PO₄ containing 3 M KI as the catholyte and 1 M H₃PO₄ as the anolyte:(A) FEs towards ECO₂R products under a range of applied potentials; (B)corresponding total current density under a range of applied potentials;

FIG. 34 shows a digital photograph of the flow channel after the ECO₂Rreaction on SF-Cu for ˜10 min in an MEA cell with 1 M H₃PO₄ containing 3M KNO₃ as the anolyte;

FIG. 35 shows ECO₂R performance on SF-Cu in an MEA cell with 1 M KOH asthe anolyte: (A) FEs toward ECO₂R products under a range of appliedpotentials; (B) corresponding total current density under a range ofapplied potentials;

FIG. 36 shows comparisons of the ECO₂R performance on SF-Cu in an MEAcell with 1 M KOH/pure H₂O as the anolyte under a range of appliedpotentials: (A, C and E) show comparisons of FEs toward C₂H₄, C₂₊, andall ECO₂R products, respectively; (B, D and F) show comparisons ofpartial current densities of C₂H₄, C₂₊, and all ECO₂R products,respectively; the reaction temperature of the ECO₂R reaction under pureH₂O is 60° C. and other ECO₂R reactions are carried out at roomtemperature;

FIG. 37 schematically depicts the MEA-cell stack containing 6 repeatingMEA cells for performing the ECO₂R reaction according to certainembodiments of the present invention;

FIG. 38 shows stability performance of ECO₂R-to-C₂H₄ on SF-Cu in an MEAcell with 1 M KOH as the anolyte at 3.2 V cell voltage according tocertain embodiments of the present invention;

FIG. 39 shows in-situ XRD measurement on SF-Cu for the ECO₂R reaction in0.1 M KOH at the 4 V cell voltage for 10 h: (A) Total current density;(B) In-situ XRD patterns, corresponding to FIG. 39A);

FIG. 40 shows in-situ XRD measurement on SF-Cu for the ECO₂R reaction in0.1 M KOH at the stepped cell voltages for 8 h: (A) Total currentdensity; (B) In-situ XRD patterns, corresponding to FIG. 40(A);

FIG. 41 shows the ECO₂R mechanism and effects of CN and tensile strainon ECO₂R by DFT calculations and experiments: (A) In-situ Raman spectraof ECO₂R on SF-Cu for 1 h in a customized flow cell with a two-electrodesystem at 4 V cell voltage according to certain embodiments of thepresent invention; (B) FEs and (C) partial current densities toward C₂H₄on SF-Cu for ECO₂R and ECOR reactions in 1 M KOH under a range of theapplied potentials; (D) A reaction energy diagram for the ECO₂R to C₂H₄on the perfect Cu and SF-Cu models via the direct *CO hydrogenation to*CHO followed by the unoccupied *CO and the *CHO dimerization pathway.

FIG. 42 shows a reaction energy diagram for the ECO₂R into the *COintermediate on the perfect Cu and SF-Cu models;

FIG. 43 shows in-situ Raman measurements on SF-Cu for the ECO₂R reactionin 0.1 M KOH at the different cell voltages;

FIG. 44 shows the total current density of the in-situ Raman measurementon SF-Cu for the ECO₂R reaction in a flow cell with 0.1 M KOH at a cellvoltage of 4 V;

FIG. 45 shows in-situ Raman measurement on SF-Cu for the ECO₂R reactionin a flow cell with 0.1 M KOH at a cell voltage of 6 V: (A) Totalcurrent density; (B) In-situ Raman spectra for 1 h;

FIG. 46 shows ECOR performance and comparisons with ECO₂R performance onSF-Cu in the flow cell with 1 M KOH as the electrolyte: (A) FEs towardECOR products under a range of applied potentials; (B) Total currentdensity for ECOR under a range of applied potentials; (C) Comparisons ofFEs and (D) partial current densities toward C₂₊ on SF-Cu for ECO₂R andECOR in 1 M KOH under a range of applied potentials;

FIG. 47 shows a comparison in reaction energy for the ECO₂R on theperfect Cu and SF-Cu models via the direct *CO hydrogenation to *CHOfollowed by the unoccupied *CO and the *CHO dimerization pathway versusthe direct *CO hydrogenation to *COH followed by the unoccupied *CO andthe *COH dimerization pathway versus 2*CO hydrogenation to 2*CHOfollowed by *CHO dimerization pathway;

FIG. 48 shows temperature-programmed desorption (TPD) of (A) CO₂ and (B)CO on SF-Cu, Cu-250, Cu-350 and Cu-450.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendepicted to scale.

DETAILED DESCRIPTION OF THE INVENTION

It will be apparent to those skilled in the art that modifications,including additions and/or substitutions, may be made without departingfrom the scope and spirit of the invention. Specific details may beomitted so as not to obscure the invention; however, the disclosure iswritten to enable one skilled in the art to practice the teachingsherein without undue experimentation.

Turning to FIGS. 1A and 1B, under the conventional alkaline condition (1M KOH), the SF-Cu catalyst delivered ECO₂R to C₂H₄ with ˜80% FE and 568mA/cm² partial current density (j_(C2H4)) at about −0.58 V (versus areversible hydrogen electrode (RHE) throughout the text, unlessotherwise noted) in a flow cell. The impressive ECO₂R performance ofSF-Cu is explicitly linked with the manipulations of its coordinationnumber (CN) and tensile strain (FIGS. 5F, 23 and 41 ). To eliminatecarbonate formation and crossover in the alkaline electrolyte, the ECO₂Rreaction is then carried out in a flow cell with the strong acid as theelectrolyte, but the strong-acid system cannot satisfy the industriallymore promising MEA-cell architecture. Finally, pure H₂O is used as anelectrolyte to perform ECO₂R-to-C₂H₄/C₂+ compounds in an MEA cellassembled with AEM and PEM. Under pure H₂O, the SF-Cu catalyst reducesCO₂ to C₂H₄ with ˜42% FE and 300 mA/cm² total current density at ˜4.3 Vcell voltage without iR compensation. In addition, the ECO₂R is scaledup in a pure-H₂O-fed 6-MEA-cell stack. At a total current of 10 A, theFE of ˜50% toward C₂H₄ is achieved and the CO₂-to-C₂H₄ conversion is upto ˜39%. This MEA-cell-stack system could be operated stably for over1000 h, which has outperformed the conventional ECO₂R-to-C₂H₄ system.

Turning to FIGS. 2A-2E, the SF-Cu nanoparticles with an average diameterof ˜60 nm (FIG. 2A) are first prepared. Detailed preparation methods ofthe SF-Cu nanoparticles can be found in some of the examples describedhereinafter. The high-resolution transmission electron microscopy(HRTEM) and aberration-corrected high-angle-annular-dark-filed scanningTEM (HAADF-STEM) images of the SF-Cu nanoparticles reveal abundantstacking faults that intersect with each other (FIGS. 2B and 2D). Theselected area (marked with E) in HRTEM image of FIG. 2C shows that themultitudinous interlaced grain boundaries in SF-Cu contain E3 coincidentsite lattice (CSL) boundaries and form some typical five-fold twinningstructures (being twin boundaries as highlighted by yellow lines inHAADF-STEM image shown in FIG. 2E), which can induce the intrinsicstress, especially large tensile strain/stress on the surface layer. TheGPA map shown in FIG. 2G reveals the local tensile strain as large as˜0.8% around the surface exits of both twin boundaries and stackingfaults. As seen in FIG. 2F, the stepped facets are induced at thesurface exits of twin boundaries and stacking faults, giving rise tosurface Cu atoms with reduced CNs. As both the high surface tensilestrain and low CNs can lead to the high-energy active surfaces forcatalytic reactions, it suggests that the abundant stacking faults andgrain boundaries in SF-Cu induce the extraordinary ECO₂R performance inthe present invention.

To verify these structural effects on ECO₂R performance, SF-Cu isannealed at various elevated temperatures (250, 350 and 450° C.; Cu-250,Cu-350 and Cu-450) to alter their microstructures. In theory, thehigh-temperature treatment will induce rearranging atoms to reach a morethermodynamically favorable state in minimizing the total surfaceenergy. The effect of annealing on the SF-Cu in the present inventionhas been explicitly shown by in-situ heating TEM images, whichdemonstrate a decrease or even a disappearance of stacking faults andtwin boundaries in the SF-Cu at high temperatures (FIG. 3 ). After eachhigh-temperature treatment, there is no appreciable change in samplesize distributions (FIG. 4 ), while all samples remain as metallic Cu(revealed by XRD pattern on carbon paper in FIG. 7 ). Only a smallextent of surface oxidation is observed on all samples by an X-rayphotoelectron spectroscopy (XPS) measurements (FIG. 8 ). Cu K-Edge X-rayabsorption spectroscopy (XAS) is conducted to investigate the localcoordination of Cu in the SF-Cu (FIGS. 9-11 ; Table 1). X-ray absorptionnear-edge structure spectroscopy (XANES) spectra in FIG. 5A confirmedthat all the samples comprise almost a pure metallic Cu phase. Inaddition, Fourier-transformed χ(R) functions of extended X-rayabsorption fine structure (EXAFS) data in the frequency domain (R)revealed an increase in CN as increasing the annealing temperature (FIG.5B). In FIG. 5F, structural parameters from EXAFS fitting resultsfurther indicate that the CNs of Cu increase gradually (from ˜7.6 to9.9), and tensile strains decrease gradually (from ˜1.03% to 0.28%) inthe order of SF-Cu, Cu-250, Cu-350 and Cu-450 (Table 1), consistent withthe observed decrease in stacking faults and twin boundaries by in-situheating TEM as described hereinabove.

TABLE 1 D-W R- factor Enot, factor, Strain, Sample Path CN R, Å ΔR, Å(62) eV % % SF-Cu Cu—Cu 7.6 ± 2.551 ± 0.026 ± 0.009 ± 6.2 ± 1.4 1.03 0.50.004 0.002 0.001 0.8 Cu-250 9.8 ± 2.540 ± 0.021 ± 0.009 ± 3.8 ± 1.70.59 0.4 0.003 0.003 0.001 0.5 Cu-350 9.6 ± 2.535 ± 0.016 ± 0.009 ± 3.3± 1.3 0.40 0.9 0.003 0.003 0.001 0.4 Cu-450 9.9 ± 2.532 ± 0.004 ± 0.008± 2.7 ± 1.2 0.28 0.8 0.002 0.004 0.001 0.4 Cu foil 12 2.525 ± — 0.007 ±2.1 ± 1.8 — 0.005 0.001 0.5 CN: coordination number; R: bond length; o:Debye-Waller factor.

To confirm the probability of surface Cu atoms with low CNs, the leadunderpotential deposition (Pb UPD) is used to identify the exposedfacets of SF-Cu, which are Cu (111) and Cu (100) (FIG. 12 ). Then, theatomic structure simulations are carried out to show the possible CNs ofCu atoms on the exposed facets (111 and 100) of SF-Cu (FIGS. 13-16 ).The perfect Cu (111) plane is composed of the surface atoms with a CN of9 (FIG. 13A), while other CNs (8, 7, 6 and 5) are also possible,depending on different sliding ways (FIGS. 13B-D and 14A-F). Similarly,the perfect Cu (100) plane contains surface atoms with a CN of 8 (FIG.15A) and atomic sites with lower CNs include 7, 6, 5 and 4 (FIGS. 15B-Dand 16A-D). As a result, CNs of Cu atoms on the SF-Cu surface vary from9 to 4 due to abundant stacking faults and interlaced grain boundaries.

SF-Cu shows the best ECO₂R performance and the highest FEs toward C₂H₄and C₂+ in the flow cell among all the samples under 1 M KOH electrolytecondition (FIGS. 5C and 17-20 ). Specifically, for SF-Cu, the peak FEtoward C₂H₄ is up to ˜80% at about −0.58 V, at which j_(C2H4) reaches˜568 mA/cm². The half-cell energy efficiency (EE_(half-cell)) of C₂H₄ isup to ˜51%. With an increase in treatment temperature, the samples showa noticeable decline for ECO₂R activity (FIG. 21 ). Such effect isapparent on ECO₂R-to-C₂H₄/C₂₊, presumably due to higher CN and lowertensile strain at high temperatures (FIGS. 5D, 5E and 22 ). It promptsto study the relationships between the CN, tensile strain, and ECO₂Rperformance. In FIG. 5F, the tensile strain and CNs show a strong linearcorrelation with the change in the annealing temperature because of therearrangement of atoms (black line). More importantly, a monotonicincrease in the partial current density (j_(C2+), j_(C2H4) orj_(without H2), in which “j_(without H2)” refers to partial currentdensity of all ECO₂R products) is observed with a decline of the CN andan increment of the tensile strain (FIGS. 3F and 23 ). In other words,the partial current density (j_(C2+), j_(C2H4) or j_(without H2)) showsa strong linear correlation as a function of the tensile strain and CNs.In contrast, the function of the tensile strain and CN shows a lowcorrelation to the partial current density of the competing reaction(j_(H2)) (FIG. 24 ).

Additionally, to decouple the effect of oxidation state (Cu⁺/Cu²⁺) onthe ECO₂R performance, an oxide-derived Cu based on SF-Cu is preparedand characterized (FIGS. 25-27 ). Compared with SF-Cu, the oxide-derivedCu barely show any improvement in ECO₂R performance in terms of eitherFEs or current densities (FIGS. 28-30 ), which suggests thatoxide-derived Cu (or the oxidation state thereof) is not a crucialfactor that determines the ECO₂R performance in the present invention,as opposed to some previous findings. After excluding the convolutingeffects of the sample size, crystal structure and oxidation state of Cu(FIGS. 4, 7 and 30 ), it is concluded the linkage of the low CN and hightensile strain with the high ECO₂R activity in SF-Cu.

Carbonate formation caused by alkaline and neutral electrolytes such asKOH and KHCO₃ for ECO₂R is fatal to the GDE and electrolysis systemstability. Some previous studies proposed some strategies to eliminatecarbonate formation, but those resulted in severe energyconsumption/penalty. A cation, e.g., potassium ion (K⁺), augmentingstrategy based on the high-performance SF-Cu catalyst in strong acidiccondition in a flow cell is assembled with a PEM (Nafion 117) to improveECO₂R reaction kinetics is provided.

Initially, SF-Cu GDE is directly used as the cathode to perform ECO₂R ina flow cell with 1 M H₃PO₄ as the electrolyte. No ECO₂R product isobserved, except H₂ (FIG. 31(A). Accordingly, a buffer layer isassembled on the SF-Cu GDE to slow the out diffusion of OH⁻ and K⁺ fromSF-Cu surface, in order to enrich potassium ion concentration andincrease local pH on said surface (FIGS. 31C and 31D). The buffer layercan be a cross-linked microporous polymethyl methacrylate (PMMA) layerand assembled on the SF-Cu GDE (SF-Cu/PMMA). However, a similar resultshows no ECO₂R product on SF-Cu/PMMA (FIG. 31B). According to thecation-augmenting strategy, when a high concentration of potassium ions(3 M KCl) is added into 1 M H₃PO₄ as the catholyte, and 1 M H₃PO₄ isused as the anolyte, SF-Cu/PMMA shows ˜40% C₂₊ FE (˜28% toward C₂H₄,˜10% toward C₂H₅OH and ˜2% toward CH₃COOH) (FIG. 32A) and a totalcurrent density of ˜360 mA/cm² at −1.2 V (FIG. 32B). Instead of KCl,when KI is used as a source of potassium ions, the C₂₊ FE is improved to˜48% (˜33% toward C₂H₄, ˜14% toward C₂H₅OH and ˜1% CH₃COOH) (FIG. 33A)with a total current density of ˜345 mA/cm² at ˜1.1 V (FIG. 33B).Overall, the present SF-Cu catalyst shows higher FEs toward C₂H₄ and C₂₊for ECO₂R than any conventional acidic system such as that disclosed inHuang et al. (2021) (Table 2), due to an improved catalyst structure andmorphology in the present SF-Cu catalyst.

TABLE 2 Total C₂₊ Potential C₂H₄ Product (V vs. j_(C2H4) Faradaicj_(C2+) Faradaic Stability Catalyst Electrolyte RHE) (mA/cm²) Efficiency(mA/cm²) Efficiency (h) Ref. SF-Cu 1M ~−0.58 ~569   ~80% ~607 ~85.48% <1— in the flow KOH cell 1M ~−1.1 ~114   ~33% ~167 ~48.57% <4 (PresentH₃PO₄ + Invention) 3 M KI// 1M H₃PO₄ 1M ~−1.2 ~101   ~28% ~147 ~40.71% —H₃PO₄ + 3M KCl//1M H₃PO₄ SF-Cu 1M ~3.2 ~134   ~40% ~196 ~58.85% <4 insingle KOH Cell MEA cell Voltage (Present Pure ~4.3 ~129   ~43% ~155~51.58% — Invention) H₂O Cell Voltage MEA cell Pure ~25 ~167   ~50% —— >1000 stack: 6 H₂O Cell MEA Cells Voltage (Present Invention)Cu₂S/Cu-V 1M −0.93 ~84.8   21.20% ~223.2   55.80% — Zhuang (Cu- KOH etal. Vacancy) (2018) Cu 1M −0.79 ~140   45.60% ~215     70%   4 Ma et al.nanoparticles KOH (2016) Cu-DAT 1M −0.6 ~75   38.20% ~137.83   70.20% —Hoang wires KOH et al. (2017) Cu dimer 1M −1.07 262     45% N/A N/A ~138Nam et al. distorted KOH (2018) HKUST-1 Nanoporous 1M −0.67 256   38.60%411     62% ~2.1 Lv et al. Cu KOH (2018) CuAg wire 1M −0.68 172   55.20%265   85.10% — Hoang et al. Alloys KOH (2018) Cu wires 1M −0.6 ~74  38.20% ~137   70.20% — KOH Ag_(0.14)/Cu_(0.86) 1M −0.67 80   ~32% 195a. 78% ~2 Li et al. KOH (2019) 1M −0.84 75   ~25% 210 a. 70% — KHCO₃Graphite/ 7M −0.55 55-70   ~70% 60-81   ~81%   150 Dinh CNP_(S)/Cu/PTKOH et al. FE (2018) 25 nm Cu 3.5M −0.67 ~473   ~65% ~608   ~81% — KOH +5M KI 25 nm Cu 10M −0.54 219     66% 275     83% <0.5 KOH Cu₄O₃-rich 0.5m −0.59 126   42.30% 183.9   61.30%   24 Martić´ catalyst Cs₂SO₄// etal. 2.5M (2019) KOH Cu₂O films 1.0M −0.74 122     67% — — <0.6 AnastasiaKOH dou et al. (2019) Cu-F 0.75M −0.89 1040     65% 1280     80% — Ma etal. KOH (2020) 1.0M −0.75 720   ~60% 996   ~83% — KOH 2.5M −0.54 480    60% 672     84% — KOH C/De-alloyed 1.0M ~−1.5 320     80% — —   50Zhong et al. Cu-A1/PTFE KOH (2020) Surface 3M −0.68 — — 336     84% —Kibria Reconstructed KOH et al. Cu (2018) Tetrahydro- 1.0M −0.83 230    72% ~261   ~82% — Li et al. bipyridine- KHCO₃ (2020) functionalizedCu MEA 0.1M b. 3.65/5 b.     64% — —   195 KHCO₃ Cell 384/5 VoltageIonomer- 7M −0.91 930     60% 1210   ~92% — Arquer coated Cu KOH et al.(2020) MEA 0.1M b. 3.9/x b.   ~55% — —   60 KHCO₃ Cell 550/x Voltage Cu(100) 7M −0.67 217   ~70% 280     90% — Wang KOH et al. (2020) MEA 0.15Mb. 3.7/5 b.   ~60% — —   70 KHCO₃ Cell 192/5 Voltage Polyamine- 1M −0.97311     72% 389     90% <3 Chen incorporated KOH et al. Cu 5M −0.62 c. —    84% — — — (2021) KOH 10M 0.47 ~28     87% — — — KOH 0.8:0.2Cu/Ag 1M−0.72 159    48.1% 287     87%   100 She et al. KOH (2020) MEA- 0.5M b.3/1 b. 106/1     48% b. 136/1     62%   150 0.8:0.2Cu/Ag KOH 0.8:0.1Cu/1M −0.70 196     45% 327     75% — Ni—N—C KOH Cu/CAL 1M ~−1.34 276  ~23% 480     40%   12.5 Huang H₃PO₄ + et al. 3M (2021) KCl//1M H₃PO₄a. A few percent of the propanol is not calculated. b. The denominatoris the area of the electrode. x. The area of the electrode is notspecified. c. The current density is not missing. —. N/A

Considering the practical viability, an industrially more applicable MEAcell is initially assembled with Nafion membrane in acidic media toperform the ECO₂R reaction. To enrich K⁺ on the SF-Cu surface, 1 M H₃PO₄containing 3 M KNO₃ is used as the anolyte. K⁺ and H⁺/H₃O⁺ in theanolyte would pass through the Nafion membrane to the SF-Cu surfaceunder the electric field. In principle, K⁺ would promote ECO₂R whileH⁺/H₃O⁺ would serve as the proton source. Although some ECO₂R productssuch as CO and C₂H₄ are formed during this initial testing, the ECO₂Rreaction is shut down after a few minutes, and hydrogen evolutionreaction (HER) became dominant. It is due to a continuous K⁺ flow fromanode to cathode causing severe carbonate precipitation in the flowchannel, which blocks CO₂ transport (FIG. 34 ). To solve this problem,pure water is used as the electrolyte for ECO₂R reaction in the presentMEA cell. One of the main problems of using MEA electrolyzer is how tomaintain high local pH on the cathode catalyst's surface for efficientECO₂R reaction. When pure water is used as an electrolyte, PEM isemployed for H⁺/H₃O⁺ transfer from the anode, and AEM is added betweenthe cathode and PEM (FIG. 6A). Under a forward bias mode, H₂O as theproton source will participate in the ECO₂R reaction at the cathode, andit will be oxidized into O₂ at the anode (FIG. 6C). The remaining OH⁻ atthe cathode and H⁺ at the anode will transport through AEM and PEM,respectively, forming H₂O at the interface of AEM and PEM (Eq. 3-5),which can effectively increase the local pH on the surface of thecathode catalyst. Although a small amount of CO₂ can dissolve in pureH₂O to form H₂CO₃ (Eq. 6), the alkaline AEM and acidic PEM willeffectively suppress H₂CO₃ formation and shift the equilibrium reactionto the left.

Cathode: 2CO₂+8H₂O+12e ⁻→C₂H₄+12OH⁻  (3)

Anode: 6H₂O→3O₂+12H⁺+12e ⁻  (4)

At the interface: 12OH⁻+12H⁺→12H₂O  (5)

CO₂ dissolution: CO₂+H₂O⇄H₂CO₃  (6)

Moreover, due to the absence of cations at the cathode to maintain theelectrical neutrality of pure water, CO₂ cannot react with theelectrogenerated OH⁻ to form carbonate and there will be no carbonatecrossover problem. H₂O can pass through both AEM and PEM. Thus, H₂O asthe proton source is sufficient for the cathodic reduction reaction.

In certain embodiments, when the total cathode electrode area is about30 cm², the flow rate of the CO₂ inlet will be about 30 sccm.

In certain embodiments, all ECO₂R reactions are conducted at a reactiontemperature of about 60° C., and Ti fiber felt sputtered by Pt (Pt/Ti)is selected as the anode electrode.

In certain embodiments, Sustainion X37-50 is selected as AEM, and Nafion117 is selected as PEM for electrogenerated OH⁻ and H⁺ ion exchangemembranes, respectively.

In other embodiments, bipolar membrane can be used as the AEM/PEM.

Preferably, Sustainion X37-50 and Nafion 117 are respectively selectedas AEM and PEM over bipolar membrane in assembling the present MEA cellsystem.

In certain embodiments, the present MEA cell system includes a cathodeselected from SF-Cu GDE and an anode selected from Ti fiber feltsputtered by Pt (Pt/Ti), where between the cathode and anode there is acombination of the AEM and PEM separating the cathode from the anodesuch that the cathode is in contact with the AEM while the anode is incontact with the PEM.

To lower the pure H₂O activation overpotential, the ECO₂R reaction onthe SF-Cu in the present MEA cell is carried out at a temperature not tosuppress ECO₂R and make HER dominant under a galvanostatic mode. Incertain embodiments, the temperature sufficient to induce ECO₂R and notto make HER dominant under the galvanostatic mode is about 60° C. (FIG.6B).

In FIG. 6B, at a total current density of 300 mA/cm², the ECO₂Rselectivity reaches the peak up to ˜66% FE, including ˜52% FE toward C₂₊(C₂H₄ FE of ˜43%, C₂H₅OH FE of ˜6%, CH₃CH₂CH₂OH FE of 2% and CH₃COOH FEof ˜1%). The cell voltage is ˜4.3 V without iR compensation. Withoutcounting the energy consumed by the reaction temperature, the proposedpure-H₂O-fed MEA-cell architecture delivers a full-cell energyefficiency (EE_(full-cell)) of ˜18.2%. The product analysis shows thatthe peak FEs and partial current densities of ECO₂R products in theproposed pure-H₂O-fed MEA system are even comparable to those in the MEAcell with 1 M KOH (FIGS. 35 and 36 ). The pure-H₂O-fed MEA cell cancircumvent the theoretical-CO₂-utilization limit for the ECO₂R reactionby thoroughly eliminating the carbonate formation and crossover.

In view of the superior ECO₂R performance on SF-Cu in the proposedpure-H₂O-fed MEA cell system, an MEA-cell stack system containing 6 MEAcells (FIGS. 6C and 37 ) is assembled and tested to evaluate itsdurability and practicality. At a total current of 10 A, six sets ofSF-Cu GDEs with a total geometrical area of 30 cm² deliver a FE of ˜50%toward C₂H₄ (FIG. 1B). The 6-MEA cell stack system can remain stable formore than 1000 hours with a full-cell-stack voltage between 25 and 27 Vwithout iR compensation (˜4.4 V cell voltage for each set of the 6 MEAcells as shown in FIG. 6D). In contrast, the stability of ECO₂R on SF-Cuin an MEA cell with the alkaline condition is even less than 4 h (FIG.38 ). The 6-MEA cell stack system can deliver up to ˜39% CO₂-to-C₂H₄conversion, and no GDE flooding is observed after 1000-h operation. Thissignificant difference in performance might be due to an elevatedreaction temperature (˜60° C.) which allows a small amount ofaccumulated H₂O on the GDEs to be discharged more quickly along with thesteam.

In certain embodiments, the pure-H₂O-fed MEA-cell stack system isfurther incorporated with an integrated circuit for monitoring ECO₂Rreaction, e.g., Arduino development, an inset in FIG. 6D. Each cell inthe system shows an almost identical voltage throughout the 1000-hmeasurement, except for some fluctuations at the first 100 h,demonstrating the possibility of the MEA-cell stack for the stable ECO₂Rat the industrial level.

Additionally, in-situ X-ray diffraction (XRD) measurements in a flowcell with a two-electrode system to assess the stability of the SF-Cucatalyst are performed, and the results are shown in FIG. 39 . Thecrystal structure of SF-Cu is proved to be stable during the ECO₂Rreaction at different cell voltages (FIGS. 39 and 40 ). In conclusion,an overall ECO₂R-to-C₂H₄ performance of SF-Cu in the flow cell, MEA celland MEA-cell stack, outperforms the most reported alkaline, neutral andacidic ECO₂R performance in other conventional systems (FIG. 1A andTable 2). More importantly, the stability of more than 1000 h of thepure-H₂O-fed MEA system will render the ECO₂R technology a step forwardto the industrial level.

Turning to FIGS. 41-48 , outstanding ECO₂R to C₂H₄ performances of SF-Cuand ECO₂R reaction pathway are demonstrated by density functional theory(DFT) calculations and in-situ and ex-situ measurements, where the SF-Cuin the pure H₂O system is shown to attribute to the combination of thisnew electrolysis architecture with a superior catalytic activity due tothe low CN and high tensile strain of the SF-Cu.

In the present disclosure, DFT calculations are performed on the perfectCu (111) and SF-Cu (111) models to reveal the outstanding ECO₂R to C₂H₄performance of SF-Cu. To amplify the impact trend from CN and tensilestrain, the unit cell of the SF-Cu model is expanded with a factor of1.1, meaning 10% tensile strain, and CN of the SF-Cu model is set to 7.The reaction energy of CO₂-to-*COOH at the SF-Cu surface is 0.39 eV(FIG. 42 ), much lower than that of the perfect Cu (0.75 eV).Subsequently, the *COOH could be easily converted into *CO due to thenegative reaction energies for the perfect Cu and SF-Cu model. Asdescribed herein, the *CO intermediate for the ECO₂R on SF-Cu isobserved by in-situ Raman measurements at different potentials (FIGS.41A and 43-45 ). The peaks located in the range of 270-360 cm⁻¹ arerelated to the Cu—CO frustrated rotation and Cu—CO stretch. The peaks atthe 1900-2200 cm⁻¹ can be ascribed to the C≡O stretch of thesurface-absorbed CO, including atop-bound CO and bridge-bound CO. Thevibration of C—H is also observed in the region from 2700 to 3000 cm⁻¹,which can be derived from hydrogenated intermediates (such as *CHO,*COCHO, etc.). A more precise assignment of these peaks is highlychallenging due to the complexity of hydrogenated intermediates ofECO₂R.

The general assumption is that C—C coupling starts with *CO. Thesubsequent dimerization reaction, however, is not verified. If the *COdimerization to *OCCO is considered the main pathway for C—C coupling,j_(C2H4)/j_(C2+) (productivity) of the electrocatalytic CO reduction(ECOR) to C₂H₄/C₂₊ on SF-Cu should be higher than that ofECO₂R-to-C₂H₄/C₂₊. To verify this assumption, direct *CO dimerization isdemonstrated by carrying out an ECOR on SF-Cu due to the high COcoverage. If the assumption is verified, one would expect the FE towardsC₂H₄/C₂₊ be higher than ECO₂R. Interestingly, SF-Cu shows a lowerj_(C2H4)/j_(C2+) for the direct ECOR (FIGS. 41B, 41C and 46 ),indicating that the *CO dimerization to *OCCO may not be the main C—Ccoupling pathway for the ECO₂R on SF-Cu. Then, two hydrogenation pathsof *CO (*CO-to-*CHO and *CO-to-*COH) are calculated (FIG. 47 ). The*CO-to-*CHO hydrogenation has less reaction energy than that of*CO-to-*COH. FIG. 41D shows that SF-Cu decreases the reaction energy of*CO-to-*CHO hydrogenation from 0.56 to 0.30 eV (FIG. 41D). Thus, twopossible pathways are proposed in the present disclosure, which are, theunoccupied *CO hydrogenation into *CHO to form 2 *CHO (*CHO+*CHO) andthe direct coupling of the unoccupied *CO and *CHO to form *COCHO. Highuphill reaction energy is required to form two *CHO on the perfect Cuand SF-Cu (FIG. 47 ), which indicates that C—C coupling by *CHOdimerization is not favorable. In contrast, the coupling of *CO and *CHOrequires lower reaction energy, and the reaction energy of the couplingof *CO and *CHO to form *COCHO at the SF-Cu surface (0.77 eV) is lessthan that of the perfect Cu (0.88 eV). The subsequent hydrogenation of*COCHO-to-*COCH₂O is exergonic for the perfect Cu and SF-Cu. Hence, thehydrogenation of *CO-to-*CHO followed by the subsequent coupling of theunoccupied *CO and *CHO to *COCHO to *COCH₂O should be the mostfavorable pathway for the C₂H₄ formation. The DFT results show thatSF-Cu would electro-catalyze CO₂ reduction to C₂H₄ more easily than theperfect Cu from the thermodynamics.

In addition, temperature-programmed desorption (TPD) measurements of CO₂and CO show that the CO₂/CO adsorption capacities of samples decreasewith an increase in treatment temperature of samples(SF-Cu>Cu-250>Cu-350>Cu-450) (FIG. 50 ). The surface Cu atoms with thelower CNs tend to bond/adsorb more CO₂/CO to compensate for the lack ofcoordination, which would accelerate ECO₂R reaction kinetics. It isbelieved that the above thermodynamic and kinetic advantages areascribed to the effects of the low CN and high tensile strain of SF-Cu.

In accordance with various embodiments of the present invention, it isevident that the abundant stacking faults and grain boundaries correlateto the low CNs and high tensile strain in SF-Cu, creating high-energyactive surfaces for ECO₂R to C₂H₄. It suggests a linkage of the lower CNand higher tensile strain with the higher ECO₂R activity. Based on thepresent SF-Cu and proposed MEA electrolysis architecture, the ECO₂Rreaction is efficiently performed under pure water, eliminating thecarbonate formation and crossover, and thus circumventing the CO₂utilization limit and prolonging the ECO₂R system stability. Inaddition, the scale-up of ECO₂R on SF-Cu in a pure-H₂O-fed MEA-cellstack is demonstrated. FE up to 50% towards C₂H₄ is achieved withCO₂-to-C₂H₄ conversion of ˜39% at a total current of 10 A, with a systemstability in terms of constant output over 1000 h. In certainembodiments, to further enhance energy efficiency of the system,selectivity of products can be improved and operating voltage thereofmay be decreased. It is believed that pure-H₂O-fed ECO₂R-to-C₂H₄ in theproposed MEA architecture injects new vitality into the ECO₂Rtechnology.

EXAMPLES

(A) Chemicals

Deuterium oxide (D₂O, 99.9 at. % D, 151882),3-(Trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt (TSP, ≥98.0%(NMR), 269913), Nafion™ solution (5 wt. %, 274704),Polytetrafluoroethylene preparation (PTFE solution, 60% in H₂O, 665800),Oleylamine (70%, 07805), Copper(I) chloride (CuCl, 97%, 212946),n-hexane (C₆H₁₄, 99%, HX0293), Octadecylamine (≥99%, 305391),Trioctylphosphine (90%, 117854), squalane (96%, 234311), Potassiumhydroxide (KOH, 99.99%, 306568), Phosphoric acid (H₃PO₄, 85%, 345245),Potassium nitrate (KNO₃, 99.0%, 221295), Lead(II) nitrate (Pb(NO₃)₂,≥99%, 228621), Potassium iodide (KI, 99%, 221945) and Potassium chloride(KCl, 99.0-100.5%, P3911) were purchased from Sigma Aldrich. Potassiumhydroxide (KOH, ≥85.0%), Nickel foam (2 mm thickness, 99.9%), andTitanium fiber felt (0.25 mm thickness, 99.9%) were purchased fromSinopharm Chemical Reagent Co., Ltd. (China). Nitric Acid (HNO₃,pH=−1.0, 70%, A200), and Isopropanol (C₃HsO, IPA, ≥99.5%, 3776) werepurchased from Fisher Scientific. The anion exchange membrane (FumasepFAA-3-PK-75), gas diffusion layer (carbon paper, GDE, Sigracet 39 BB),and Nafion® 117 membrane (591239) were purchased from FuelCellStore. Thealkaline ionomer solution (5% in ethanol, Sustainion XA-9) and anionexchange membrane (Sustainion X37-50) were purchased from DioxideMaterials.

(B) Catalysts Preparation

In a typical synthesis, 0.05 g of CuCl and 0.1 g of octadecylamine weredissolved in 1 mL of squalane at 80° C. under the Ar atmosphere and keptthis temperature for 0.5 h to form the Cu-based stock solution. 10 mL ofoleylamine and 0.5 mL of trioctylphosphine were added to a flask andheated to 200° C. under the Ar atmosphere with intense magneticagitation. Then, the Cu-based stock solution was quickly injected intothe above 200° C. oleylamine solution and kept at this temperature for 5h. After natural cooling, the resulting sample was collected bycentrifugation and washed several times with n-hexane. Finally, thesample was blown dried with Ar gas at room temperature. Due to thestepped-facet surface, the sample was denoted as SF-Cu.

To study the structure-activity relationship of SF-Cu for theelectrocatalytic CO₂ reduction, the SF-Cu samples were annealed atvarious temperatures (250, 350, and 450° C.; Cu-250, Cu-350, and Cu-450)in the tube furnace for 2 h under a mixed gas (H₂/Ar: 5 v/v %; 200 sccm(standard cubic centimeters per minute)) to prevent oxidization. Inaddition, the oxide-derived Cu was prepared by directly calcining SF-Cuat 450° C. in the air for 2 h.

(C) GDEs Fabrication

For the flow cell and MEA cell measurements under the alkalinecondition: Cathode GDEs were prepared on conventional carbon paper. Thecatalyst was dispersed in a mixed solution containing H₂O, IPA (1:4 v/v)and some alkaline ionomer solution (5 wt. % vs. catalyst, SustainionXA-9) by the sonication for 1 h to form a 1 mg/mL catalyst ink. GDEswere fabricated by spraying the ink onto the carbon paper with amicroporous carbon gas diffusion layer with the loading of ˜1 mg/cm²,followed by drying at 120° C. in a vacuum for 1 h before use (SF-CuGDE). Anode electrode was the mixture of IrO_(x) and RuO_(x) supportedcarbon paper.

For the flow cell and MEA cell measurements under the acidic condition:The alkaline ionomer was replaced with Nafion™ solution. PMMA containingPTFE solution was spray-coated on the SF-Cu GDE as the cathode GDE(SF-Cu/PMMA), and the mixture of Pt-supported Ti fiber felt (Pt/Ti) wasused as the anode electrode. Pt was sputtered on the Ti fiber felt usinga pure Pt target in an Ar environment (5×10³ Torr) in a magnetronsputtering system.

For MEA measurements under pure H₂O, the SF-Cu GDE and Pt/Ti GDE weredirectly used as the cathode and anode electrodes, respectively.

(D) Electrocatalytic CO₂/CO Reduction

Electrochemical tests in the flow cell and MEA cell were performed usingan electrochemical workstation (CHI 660E) connected to a current booster(CHI 680C), except for the MEA-cell stack. The mass flow controller(MFC, Alicate Scientific MC) was used to control the CO₂ flow rate. Theflow rate of the electrolyte stream was 5 mL/min controlled by aperistaltic pump unless otherwise noted. The area of the cathode in theflow cell and MEA was 1 cm×1 cm unless otherwise noted. All ECO₂Rmeasurements were carried out at room temperature unless otherwisenoted. For all flow cell measurements, the Hg/Hg₂Cl₂ (SCE, saturatedKCl) was used as the reference electrode, and all cathode potentials(vs. Hg/Hg₂Cl₂) were converted to RHE scale via the following equation:

E _((RHE)) =E _((Hg/Hg) ₂ _(Cl) ₂ ₎+0.241+0.0591×pH+iR

where R is the resistance between the cathode and reference electrodesmeasured by electrochemical impedance spectroscopy (EIS) with afrequency range from 105 Hz to 0.01 Hz at open circuit potential. Forall MEA measurements, the full-cell voltages were directly presentedwithout iR compensation.

Under the alkaline condition: For the flow cell measurements, 1 M KOHwas used as the electrolyte, and the anion exchange membrane (AEM,Fumasep FAA-3-PK-75) was used to separate the catholyte and anolytecompartments. The CO₂/CO was supplied to the cathode at a flow rate of30 sccm. For ECO₂R in an MEA cell with the alkaline condition, 1 M KOHwas used as the anolyte, the cathode and anode GDEs were separated by anAEM (Sustainion X37-50).

For scale-up MEA-cell stack measurements, an integrated circuit based onthe Arduino development board (UNO R3, A000066) was used as an aidedmonitoring system connected with the CoolTerm serial port terminalapplication tool. All electrocatalytic CO₂ reduction measurements in thescale-up MEA-cell stack were carried out by the customized Varied DCpower supply (1000 W). The flow rates of the anolyte and CO₂ were 15mL/min and 30 sccm, respectively. The reaction temperature was 60°.

(E) Products Analysis

For both of the electrocatalytic CO₂ and CO reduction, the gas andliquid products were quantified by the gas chromatograph (GC, GC-2030,Shimadzu) and nuclear magnetic resonance (NMR, ECZ500R, 500 MHz, JEOL)spectroscopy. GC was equipped with two thermal conductivity detectors(TCD) for H₂, O₂, N₂, He, CO and CO₂ signals and a flame ionizationdetector (FID) for CH₄, C₂H₄ and C₂H₆ signals. GC was composed of packedcolumns of two Porapak-N, a Molecular sieve-13X, a Molecular sieve-5A, aPorapak-Q and an HP-PLOT AL/S column, and employed He (99.999%) and N₂(99.999%) as the carrier gases. To calibrate the CO₂ flow rate at theoutlet of the cell (f_(CO2)), He used as the internal standard was fedat 10 sccm and mixed with the outlet gas stream of the cell beforeinjecting to GC (20). The FEs of gas products were calculated by thefollowing equation:

${{FE}(\%)} = {N_{x} \times F \times m_{x} \times \frac{f_{CO_{2}}}{j_{total}} \times 100\%}$

where N_(x) is the number of electrons transferred for the specificproduct (x), F is the Faradaic constant, m_(x) is the molar fraction ofthe specific product (x) determined by GC, f_(CO2) is the molar flowrate of the CO₂, and j_(total) is the total current density.

The liquid products were analyzed by 500 M Hz ¹H NMR spectroscopy(ECZ500R, JEOL) with water suppression. TSP and D₂O were used as thereference standard and lock solvent, respectively. The FEs of liquidproducts were calculated by the following equation:

${{FE}(\%)} = {N_{x} \times F \times \frac{C_{x} \times V_{x}}{Q_{total}} \times 100\%}$

where N_(x) is the number of electrons transferred for the specificliquid product (x), F is the Faradaic constant, C_(x) is theconcentration of the specific liquid product (x) determined by ¹H NMR,V_(x) is the volume of the electrolyte, and Q_(total) is the totalcharge.

The half-cell and full-cell energy efficiencies (EE_(Half-cell) andEE_(Full-cell)) were calculated as the following equations (take oxygenevolution reaction (OER) as an example of the anode reaction and assumeit to occur with an overpotential of 0 V, E_(OER) ^(θ)=1.23 V vs. RHE)):

$\begin{matrix}{{{EE}_{{Half} - {cell}}(\%)} = {\frac{( {E_{OER}^{\theta} - E_{x}^{\theta}} ) \times FE_{x}}{E_{OER}^{\theta} - E_{C}} \times 100\%}} \\{= {\frac{( {{{1.2}3} - E_{x}^{\theta}} ) \times FE_{x}}{{{1.2}3} - E_{C}} \times 100\%}}\end{matrix}$ $\begin{matrix}{{{EE}_{{Full} - {cell}}(\%)} = {( {E_{OER}^{\theta} - E_{x}^{\theta}} )/E_{{Full} - {cell}} \times {FE}_{x} \times 100\%}} \\{= {\frac{{{1.2}3} - E_{x}^{\theta}}{E_{{Full} - {cell}}} \times {FE}_{x} \times 100\%}}\end{matrix}$

where E_(OER) ^(θ) and E_(x) ^(θ) are the thermodynamic potentials (vs.RHE) for OER and the electrocatalytic CO₂ reduction to the product (x),respectively, FE_(x) is the FE of the product (x), E_(C) is the appliedpotentials at the cathode, and E_(Full-cell) is the cell voltage of theMEA system.

CO₂ conversion was calculated by the following equations:

$f_{x} = \frac{Q_{total} \times {FE}_{x}}{F \times N_{x} \times t \times A}$${{CO}_{2}{Conversion}(\%)} = {( {f_{CO} + f_{{HCOO}^{-}} + f_{{CH}_{4}} + {2f_{C_{2}H_{5}{OH}}} + {2f_{{CH}_{3}{COO}^{-}}} + {3f_{1 - {C_{3}H_{7}{OH}}}}} ) \times \frac{A}{f_{{CO}_{2}}} \times 100\%}$

where f_(x) is the molar rate of the product (x) formation, t is theelectrolysis reaction time, and A is the geometric area of theelectrode.

(F) In-Situ Electrochemical Raman Measurements

In-situ Raman measurements were carried out by a customizedspectro-electrochemical flow cell fabricated with a sapphire window (thethickness of 0.15±0.02 mm) in front of the cathode GDE. The Ni felt wasused as a counter electrode. The overall system was operated in atwo-electrode setup. The electrolyte (0.1 M KOH) was pumped into asapphire window at a constant flow rate of 5 mL/min by a peristalticpump over the cathode GDE, and the thickness of the electrolyte level onthe cathode surface was 1.5 mm. CO₂ was supplied to the back of thecathode GDE through the serpentine flow channel to guide the CO₂ at aflow rate of 30 seem controlled by an MFC (Alicate Scientific MC). Ramanspectra were collected under the accumulation time of 4 s andaccumulation number of 10 times by using a WITEC Confocal Ramanmicroscope with an objective (50×) and a 633 nm laser. The cell voltagewas applied in potentiostatic mode and recorded without iR compensation.

(G) In-Situ Electrochemical XRD Measurements

The customized spectro-electrochemical flow cell was employed to performthe in-situ XRD measurements operated in a two-electrode setup. Ni feltwas used as a counter electrode, 0.1 M KOH was used as the electrolyte,and the CO₂ (30 sccm) was supplied to the back of the cathode GDE. Thein-situ XRD patterns were collected on an X-ray diffractometer (RigakuSmartLab 9 kW—Advance) using Cu Kα radiation (λ=1.5418 Å) at 45 kV and200 mA. The single test time was about ˜8 min in the range (2θ) of 30°to 85°. The cell voltage was applied in potentiostatic mode and recordedwithout iR compensation.

(H) In-Situ Heating TEM Measurements

In-situ heating TEM measurements were performed on the JEOL ModelJEM-2100F at 200 kV with a Fusion Select holder (Protochips) and a holeycarbon-coated MEMS E-chip.

(I) Pb Underpotential Deposition Measurements

Relative populations of the exposed facets of Cu were probed using Pbunderpotential deposition (Pb-UPD). Pb-UPD measurements were conductedin a three-electrode single-compartment cell. A graphite carbon rod andAg/AgCl (3 M KCl) were used as the counter electrode and referenceelectrode, respectively. An L-type glassy-carbon electrode loaded thesample with a diameter of 3 mm was employed as the working electrode. AnN₂-purged 0.1 M KNO₃ with 1 mM Pb(NO₃)₂ was added with HNO₃ to adjustthe pH to 1, used as the electrolyte. Cyclic voltammetry (CV) with asweep rate of 100 mV/s was used for measurements.

(J) Temperature-Programmed Desorption Measurements

Temperature-programmed desorption (TPD) measurements of CO₂ on sampleswere conducted with an adsorption/desorption system. In a typicalexperiment, 1 cm² GDE with the catalyst load of ˜1 mg/cm² was groundinto powder, the powder was placed in a U-shaped quartz microreactor.Next, the outlet of the U-shaped quartz microreactor was connected to GC(GC-2014, Shimadzu) with a TCD detector. Afterward, the CO₂ (40 sccm)was injected into the U-shaped quartz microreactor and kept flowing for60 min, followed by flushing the sample using the He stream (40 sccm)until obtaining a stable baseline of GC. TPD measurements were thenconducted from room temperature to 800/500° C. at a ramp rate of 10°C./min, and GC would detect the desorbed CO₂ from the sample surface.

(K) DFT Calculations

All DFT calculations were performed on Vienna ab initio simulationprogram (VASP). The generalized gradient approximation (GGA) with thePerdew Burke-Ernzerhof (PBE) exchange-correlation functional was adoptedto describe the electronic exchange and correlation interactions with acut-off energy of 500 eV. The energy convergence criteria was set to be10⁻⁵ eV for self-consistent calculations, and the lattice parameterswere optimized until the convergence tolerance of force on each atom wassmaller than 0.05 eV. The 4×4×1 Monkhorst-Pack k-point mesh was used forthe Brillouin zone integration.

For the perfect Cu, the copper crystal structure was optimized with alattice constant of α=3.636 Å. For Cu-SF, the unit cell was expandedwith a factor of 1.1 and then fully relaxed until getting convergence.The lattice constant was determined to be 4.000 Å. Six-layer p(4×4)supercells of Cu (111) facet were used, with the lower three layersfixed. For all slab models, the vacuum thickness in a directionperpendicular to the plane of the catalyst was at least 15 Å to avoidthe attractions from adjacent periodic mirror images. At allintermediate states, two water molecules are added near the slab surfaceto take the effect of solvation into account.

The Gibbs free energy (ΔG) of the reaction intermediates is defined asthe following equation:

ΔG=ΔE+ΔZPE−TΔS

where ΔE is the total energy difference, ΔZPE is the difference of thezero-point energy, and TAS is the difference of entropy. Note that E(H)is half of the H₂ (g) energy under 1.013 bar at 298.15K, E(H₂O) is theenergy of H₂O (g) under 0.035 bar at 298.15 K and E(OH)=E(H₂O)−E(H). Thezero-point energy and entropy were corrected by calculating thevibrational frequencies through density functional perturbation theoryat 298.15 K.

(L) Materials Characterizations

TEM images were collected on a JEOL JEM-2100F at 200 kV.Aberration-corrected HAADF-STEM images were collected on a TFS Spectra300 at 300 kV. GPA analysis on atomic-resolution images was performedusing Digital Micrograph software to derive the lattice strain. Onlystrain perpendicular to the stacking faults and twin boundaries wasmeasured, using the lattice far from these defects as a reference (zerostrain). SEM images were taken on the field emission Tescan MAIA3. TheXRD patterns were recorded on a Rigaku SmartLab 9 kW-Advancediffractometer with Cu Kα radiation (λ=1.5418 Å). XPS spectra werecollected on a Thermo Scientific Nexsa X-ray photoelectron spectroscopyusing Al Kα radiation, and C is (284.6 eV) as a reference. The hardX-ray absorption spectroscopy measurements were conducted at thebeamline BL01C of the Synchrotron Radiation Research Center (SRRC) inHsinchu (Taiwan).

Although the invention has been described in terms of certainembodiments, other embodiments apparent to those of ordinary skill inthe art are also within the scope of this invention. Accordingly, thescope of the invention is intended to be defined only by the claimswhich follow.

INDUSTRIAL APPLICABILITY

The present invention provides a stackable MEA electrolysis cell systemthat can be operable with pure H₂O such that carbonate formation andcrossover can be eliminated. It is easy to be fabricated and scaled upor down according to industrial application and CO₂ reduction demand.The present invention is not just cost-efficient but also a moreenvironmental-friendly way to reduce CO₂. Higher yield of usefulby-products from ECO₂R reaction generated by the present invention isalso resulted.

REFERENCE

The following literatures are cited herein:

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1. A pure-H₂O-fed membrane-electrode assembly electrolysis system forelectrocatalytic CO₂ reduction to ethylene and C₂₊ compounds under anindustrial applicable continuous flow condition with at least 1000-hourlifetime, the system comprising one or more membrane-electrodeassemblies each comprising: an anode; a cathode; an anion exchangemembrane; a proton exchange membrane; a step-facet-rich copper catalystdisposed at the cathode, wherein the step-facet-rich copper catalyst hasa surface atom coordination number of 7 or lower at Cu (111) exposedfacet; and an electrolyte, the cathode being arranged in contact withthe anion exchange membrane; the anode being arranged in contact withthe proton exchange membrane; the anion exchange membrane and protonexchange membrane being arranged in contact with each other; theelectrolyte being selected from pure H₂O as proton source for theelectrocatalytic CO₂ reduction at the cathode under a forward bias modeof the system; the anion exchange membrane being selected from alkalineanion exchange membrane; and the proton exchange membrane being selectedfrom acidic proton exchange membrane.
 2. The system of claim 1, whereinthe cathode is selected from a gas diffusion electrode deposited with atleast a layer of the step-facet-rich copper catalyst.
 3. The system ofclaim 1, wherein the anode is selected from titanium liber feltsupported by one or more of platinum, iridium, ruthenium, and palladium,and any oxide or alloy thereof.
 4. The system of claim 1, wherein theelectrocatalytic CO₂ reduction is conducted at a temperature of about60° C. or lower but above room temperature.
 5. The system of claim 1,wherein the alkaline anion exchange membrane is an anion exchangemembrane made of N-methylimidazolium-functionalized styrene polymer. 6.The system of claim 5, wherein the anion exchange membrane has athickness of about 0.002 inches.
 7. The system of claim 1, wherein theacidic proton exchange membrane is a proton exchange membrane made oftetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer.
 8. The system of claim 7, wherein the proton exchangemembrane has a thickness of about 0.007 inches and an equivalent weightof about 1100 g/mol.
 9. The system of claim 1, wherein thestep-facet-rich copper catalyst has a surface atom coordination numberfrom 4 to 9 at Cu (100) exposed facets and from 4 to 7 at the Cu (111)exposed, facet.
 10. The system of claim 1, wherein the step-facet-richcopper catalyst has a surface tensile strain being within 10% increaseof an initial tensile strain thereof measured at room temperature. 11.The system of claim 1, wherein the electrolyte at the cathode isidentical to an electrolyte at the anode.
 12. The system of claim 1,wherein at least six of the membrane-electrode assemblies are stackedtogether.
 13. The system of claim 12, wherein up to about 50% ofFaradaic efficiency towards ethylene with a carbon dioxide-to-ethyleneconversion efficiency of about 39% is achieved when a total current of10 A is supplied across the at least six membrane-electrode assembliesthrough two conductive substrates sandwiching the stack of the at leastsix membrane-electrode assemblies with a total geometrical area of 30cm².
 14. A method for fabricating a pure-LO-fed membrane-electrodeassembly electrolysis system for electrocatalytic CO₂ reduction toethylene and C₂₊ compounds with at least 1000-hour lifetime, comprising:providing a step-facet-rich copper catalyst, wherein the step-facetcopper catalyst has a surface atom coordination number of 7 or lower atCu (111) exposed facet; preparing a step-facet-rich coppercatalyst-containing ink composition for forming a cathode with thestep-facet-rich copper catalyst thereon; forming the cathode with thestep-facet-rich copper catalyst thereon; preparing an anode-formingmixture for forming an anode; forming the anode from the anode-formingmixture supporting an anode material; providing an alkaline anionexchange membrane and an acidic proton exchange membrane between saidcathode and anode, the alkaline anion exchange membrane being arrangedin contact with the cathode, the acidic proton exchange membrane beingarranged in contact with the anode, and the alkaline exchange membraneand acidic proton exchange membrane being in contact with each other,thereby forming a multi-layered structure of the membrane-electrodeassembly; sandwiching one or more of the membrane-electrode assemblieswith two conductive substrates; feeding pure H₂O as an electrolyte intoa container containing the one or more of the membrane-electrodeassemblies being sandwiched between the two conductive substrates;providing a power supply to the one or more of the membrane-electrodeassemblies through the two conductive substrates; maintaining theelectrolyte at a temperature sufficient for the electrocatalytic CO₂reduction to ethylene to last fir at least 1000 hours with no dominanthydrogen evolution reaction.
 15. The method of claim 14, wherein saidproviding the step-facet-rich copper catalyst comprises: dissolvingcopper chloride and octadecylamine into squalene at about 80° C. underan argon atmosphere for about 0.5 hours until a copper-based stocksolution is formed; mixing oleylamine and trioctylphosphine underheating the mixture to about 200° C. at the argon atmosphere withvigorous agitation to form a mixture; injecting the copper-based stocksolution into the mixture at about 200° C. and maintained for about 5hours to form a reaction mixture; cooling the reaction mixturenaturally, centrifuging the cooled reaction mixture, followed by washingwith n-hexane for a few times; removing supernatant after said washingand blow drying pellet with argon gas under room temperature to obtainthe step-facet-rich copper catalyst in solid form.
 16. The method ofclaim 15, wherein said forming the cathode with the step-facet-richcopper catalyst thereon comprises: dispersing the solid step-facet-richcopper catalyst into a mixed solution containing water, isopropylalcohol and an alkaline ionomer solution; mixing the solidstep-facet-rich copper catalyst with the mixed solution by sonicationfor about an hour until the step-facet-rich copper catalyst-containingink composition is formed; coating the step-facet-rich coppercatalyst-containing ink composition onto a carbon paper with amicroporous carbon gas diffusion layer; drying the coatedstep-facet-rich copper catalyst-containing ink composition on the carbonpaper in vacuum for about an hour.
 17. The method of claim 14, whereinthe anode is formed from a titanium fiber felt supported by the anodeforming mixture comprising one or more of platinum, iridium, ruthenium,and palladium, and any oxide or alloy thereof.
 18. The method of claim14, wherein the alkaline anion exchange membrane is selected from ananion exchange membrane made of N-methylimidazolium-functionalizedstyrene polymer with a thickness of about 0.002 inches.
 19. The methodof claim 14, wherein the acidic proton exchange membrane is selectedfrom a proton exchange membrane made oftetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acidcopolymer with a thickness of about 0.007 inches and equivalent weightof 1100 g/mol.
 20. The method of claim 14, wherein at least six of themembrane-electrode assemblies are stacked with each other and sandwichedbetween the two conductive substrates; the electrolyte temperature ismaintained at about 60° C.